RFR: 8373208: Make Atomic class template constant initializable

[Kim Barrett]

Please review this change to Atomic to meet the requirements for constant
initialization in cases where the value type provides the requisite support.
This ensures that in such cases dynamic initialization won't even be
considered a possibility when initialized with constant arguments.

Testing: mach5 tier1

-------------

Commit messages:
 - constexpr Atomc ctors

Changes: https://git.openjdk.org/jdk/pull/28711/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=28711&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8373208
  Stats: 27 lines in 1 file changed: 13 ins; 1 del; 13 mod
  Patch: https://git.openjdk.org/jdk/pull/28711.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/28711/head:pull/28711

PR: https://git.openjdk.org/jdk/pull/28711